The universal set is the set of all possible elements under consideration.

You can have a universal set of all people, or all bird species, or all numbers or whatever. You can even have a universal set of all people and all bird species and all numbers as one big set.

The set of all integers;

the set of all rational numbers;

the set of all real numbers;

the set of all complex numbers.

Also their multiples - for example

the set of all multiples of 2;

the set of all multiples of 2.5;

the set of all multiples of sqrt(17);

the set of all multiples of 3 + 4i where i is the imaginary square root of -1.

false, because the complement of a set is the set of all elements that are not in the set.

The answer depends on what do you mean by "all". It could be the set of all integers, the set of all rationals or the set of all reals.

2 is the only member of both the set of all prime numbers and the set of all even numbers.